The present invention relates generally to a magnet, and more particularly to a magnet having shielding to protect the area around the magnet from stray magnetic fields originating from the magnet.
Magnets are used in diverse applications such as MRI (magnetic resonance imaging) systems. MRI systems include those employing superconductive magnets for medical diagnostics and procedures. Known superconductive MRI magnet designs include those having superconductive main coils each carrying a large, identical electric current in the same direction. The superconductive main coils create a static magnetic field within an MRI imaging volume which typically has the shape of a sphere centered within the magnet's bore where the object to be imaged is placed.
Superconductive magnets having shielding include those having superconductive shielding coils and those having a cylindrical iron shield. The superconductive shielding coils carry electric currents of generally equal amperage, but in an opposite direction, to the electric current carried in the superconductive main coils. The superconductive shielding coils are positioned radially outward from the superconductive main coils to prevent the high magnetic field created by and surrounding the superconductive main coils from adversely interacting with electronic equipment in the vicinity of the magnet. Likewise, the cylindrical iron shield is positioned radially outward from the superconductive main coils to prevent the high magnetic field created by and surrounding the superconductive main coils from adversely interacting with electronic equipment in the vicinity of the magnet.
Short magnets with a uniform field, as would be used for MRI, may achieve field uniformity by use of superconductive bucking coils, permanent magnet rings, and iron rings placed radially inward of the main coils. Iron rings are low in cost, but, when positioned in the magnet bore, they must be temperature controlled to maintain a highly uniform field. In addition, the iron rings may interact with other MRI imaging components. Shielding short magnets with a thick iron shield adversely affects field uniformity.
Superconductive magnets having superconductive shielding coils use nearly twice as much superconductor as unshielded magnets. Short magnets with superconductive bucking coils for field correction use more superconductor than longer magnets without the bucking coils. What is needed is a short magnet having shielding which is not as expensive as known designs and which is without the problems associated with iron rings in the warm magnet bore.